A gamecock is a type of rooster and game fowl, a type of chicken with physical and behavioral traits suitable for cockfighting. The first use of the word gamecock, denoting use of the cock as to a “game”, a sport, hobby, or entertainment, was in 1646[1] after the term “cock of the game” was used by George Wilson, in the earliest known book on the sport of cockfighting in The Commendation of Cocks and Cock Fighting in 1607. Cockfighting may be 5,000 or more years old. Game fowl appear more like their wild ancestor the Red Jungle Fowl, a shy wild chicken from forests in South Central and Southeastern Asia, than do other domestic chickens. This is because game fowl have been bred to promote the fittest individuals rather than to promote mutations which increase egg production, feed conversion for meat or fancy chickens bred for exhibition. The territorial instinct among sexually mature males is the driving force behind their desire to dominate and eliminate other males that would compete for breeding rights in their territory. Roosters will fight each other to the death regardless of human contact; it is their natural instinct to fight due to the condition of being alpha males, cockfighting, like most blood sports, is illegal in most of the world but exceptions exist. Although cockfighting is illegal in the United States, in Puerto Rico it is very popular.[citation needed]

"Gameness" describes not only initial aggressiveness in the males but also the requirement to remain aggressive even when severely injured as this is necessary to win a fight in "the pit", coliseum, etc. Hens typically also show above average aggression compared to other domestic breeds and introduction of a hen into a confined space with another will result in a fight and introduction into a group may result in injury to the newcomer. Other breeds of domestic chickens which are bred primarily for egg and meat production have been bred over many generations to cohabitate on farms or other smaller pieces of land and gameness has largely been bred out of them.

Game fowl actively used in cockfighting are not a recognized breed by poultry associations, although they are known by many names which could be confused with breed names. Those names are most often the name of a successful breeder or a name a successful breeder has applied to his fowl. Some of those names have come to be associated with fowl having certain physical characteristics, often some combination of feather color, shank color and comb type although rarely do breeders actually breed game fowl to breed true (so as to all look alike). They are generally bred from unrelated individuals, known for their fighting style. The broodcock often having been a winner in the pit and from hens that are close relatives of winners.

Pullets (young females) and stags (young males) are typically free ranged because it promotes a better diet, better exercise opportunities and better protection against communicable disease although with greater risk from predators. When the stags on free range begin to fight all of the stags of that age group will require being housed separately to prevent injuries from fighting each other and preventing contact with penned cocks or cocks on tie cords. Separate housing includes separation by more than wire fencing because males will fight against a wire fence and this will result in injury to feet and legs. Sometimes sexually mature stags less than one year old are "pitted" in stag contests but more often cocks are 1 1/2 to 2 years of age before being engaging in serious competition. Cocks are selected by fitness, build, weight and by observing their sparring with another cock, often with their natural spurs covered with a soft "muff" to protect both birds from injury. Their natural spurs are sawed off to a length of about 3/8 inch which minimizes the risk of injury to people handling them and to their own kind. For serious combat, they are very rarely unarmed as most organized contests are conducted under rules allowing their being armed with metal spurs called gaffs which have a socket which fits over the reduced-length natural spur. These contests often result in fatal injuries to one of the combatants. Spectators viewing these competitions are often interested in wagering on these contests.[citation needed]

Prior to physical conditioning, a gamecock that is to be fought or shown is groomed. The comb (the red skin on top of the head) and wattles (skin under the beak) is usually trimmed at around a year old. This process is called “dubbing”. Dubbing eliminates a potential billhold the opponent may use to advantage during competition. The feathers are sometimes groomed as well. The sickle feathers of the tail may be trimmed or any long feathers that a cock might trip on during a fight. In some cultures (particularly among Cuban game fowl enthusiasts), the feather trimming is much more extensive. The feathers of the chest and the back are sometimes shorn completely off. The reason for this extensive trimming is to help prevent a bird from overheating during a longer match. The reasons for this vary among individual game fowl enthusiast. Some trim their birds according to a tradition and others do it because they believe that losing the “bulky” feathers improves mobility during a fight.

A gamecock will undergo a conditioning process in preparation for a fight. The conditioning process is referred to as the "keep" and is designed to ensure that the bird is physically and mentally fit for its upcoming match and usually last for 7–10 days. The keep includes a special diet as well as physical exercise in the hands of the responsible individual. The handling also tames the cock so that he can be handled during a fight with no attention given to his handler. Cocks that are aggressive to people are called man fighters and are not well tolerated as it makes exercising them while in the keep more difficult and a cock that turns to face his handler in the pit is usually immediately taken down by his actual opponent.

Cockfighting is a seasonal sport. From September to November, gamecocks go through their molting stage (lose old feathers and grow new ones). This is a sensitive time for the gamecock, so no fighting occurs. In exhibitions where fowl are judged for their apparent physical attributes, there are classes for several breeds which have "Game" as part of their breed name such as the Modern Game and Old English Game, as well as several bantam breeds, for example American Game Bantam, that are recognized by the American Poultry Association but are not actually used for cockfighting. The APA does not recognize what is sometimes referred to as the American Game (standard size) which is the breed used in the United States in cockfighting.

The Tufts University Men's Track and Field team created an alter ego division 1 fictional school called Jumbo State, and have designed shirts representing the Jumbo State Gamecocks combining a cock and an elephant (the Tufts mascot is Jumbo the elephant).